1. Field of the Invention
The present invention relates to a photodetection device for converting photocurrent generated in a photoelectric conversion element to voltage.
2. Description of the Related Art
A photodetection device converts photocurrent, which is generated according to an illuminance of a light entering a photoelectric conversion element such as a photodiode or a phototransistor, into a voltage for output.
Amount of the photocurrent generated in the photoelectric conversion element such as the photodiode or the phototransistor is proportional to the illuminance. One form for the output voltage from the conventional photoelectric conversion circuits for converting photocurrent to a voltage is proportional to the illuminance of the light, and another form is proportional to a logarithmic value of the illuminance of the light.
A photodetection device disclosed in JP 11-211563 A shown in FIG. 4, for example, adopts a method in which a photocurrent, which is generated in a photodiode 1 by light, is converted to a voltage by an amplifier 2 with the help of a resistor 3, to thereby obtain an output voltage as a linear equation of a magnitude of the current generated in the photodiode, permitting acquisition of a voltage proportional to an illuminance of the light.
As another example, FIG. 5 shows a photodetection device disclosed in JP 2005-241306 A in which a diode 1 is connected in series with a phototransistor 4 serving as a photoelectric conversion element, and in which an amplifier 2 and a resistor 3 are provided so that an output voltage is proportional to a logarithmic value of a magnitude of a photocurrent generated in the phototransistor, utilizing the fact that a current-voltage characteristic of the diode is exponential. The magnitude of the photocurrent is proportional to an illuminance, and hence an output in proportion to the logarithmic value of the illuminance of the light can be obtained. High resolution for detecting the illuminance can be obtained at low illuminance.
Obtaining the output proportional to the logarithmic value of the illuminance of the light is accompanied by the following problems.
First, current consumption of the photodetection device tends to be large since a current should be fed to the photoelectric conversion element continuously in order to obtain the output voltage, and since a current flowing through a light receiving element should be large enough to improve a signal-to-noise (SN) ratio to obtain sufficient sensitivity. Second, the output from the circuit is logarithmic, and hence simple and accurate signal processing using a digital circuit is difficult to be performed afterward.